The invention relates to the field of coronary implants, and in particular to a low disturbance, pulsatile, in vitro flow circuit for modeling coronary implant thrombosis.
Biocompatibility has been a major issue in the ability to use prosthetic implants in clinical settings. One such set of applications includes vascular prosthesis such as endoluminal stents or grafts to allow blood to flow either through or past a previously stenosed vascular segment. When such a foreign structure comes into contact with tissue and blood, a variety of biological consequences ensue. These reactions, ranging from thrombosis, to inflammation, to restenosis, can result in acute or long-term device failure. Not only is coagulation responsible for the obvious occurrences of acute thrombotic events, but sub-clinical levels have also been implicated as a player in the pathophysiology of restenosis through the release of chemical mediators and by providing a scaffold for the ingrowth of migrating and proliferating cells.
The thrombotic reaction is one of the earliest responses to implantation and by virtue of its potential for rapid acceleration and complete luminal occlusion, one of the most devastating. Forming clot not only serves as a scaffold for the ingrowth of migrating and proliferating cells, but as a source and reservoir for chemical mediators of these cellular events, such as platelet derived growth factor and thrombin. Elucidation and control of the thrombotic process is especially important for the continued use and development of vascular implants.
Vascular patency relies on a careful balance of chemical mediators and local fluid dynamics. With vascular injury, even as simple as the insertion of a small intravascular wire, profound micro-environmental changes ensue, altering blood flow and coaguability. A thrombus develops and propagates when the stimulatory forces cannot be balanced by the negative regulatory measures. Platelets adhere and activate at a given implantation site, potentiating the coagulation reactions by acting as an enzymatic surface and sequestering reactants both from flow and other inhibitory influences. These coagulation processes then potentiate further platelet activation directly via the production of mediators such as thrombin and indirectly by stabilizing the adherent platelets via a fibrin meshwork. Physiologically, these cellular and molecular systems interact in a highly inter-dependent manner to make thrombosis possible in the face of arterial flow conditions.
One difficulty that has limited the extensive examination of bioprosthetic thrombosis is the highly flow-dependent nature of thrombosis and lack of widely applicable flow models. Flow can affect the components of thrombosis either through physical shear dependent mechanism, such as von Willebrand's Factor dependent platelet activation, or through mass transport of cellular and molecular substances into and out of a given region. Thus, control and documentation of reproducible flows are essential to the study of the dynamically coupled cellular and protein pathways leading to implant thrombosis. Also, doing so in a controllable in vitro setting is desirable as individually and controllably perturbing the various thrombotic components is essential to studying the dynamically coupled cellular and protein pathways.
Various prior art flow systems have been developed in order to study the thrombotic process. One such method includes placing a loop partially filled with blood on a tilted turntable. As the table spins, gravity keeps the fluid at the bottom of the tube, creating flow. This method is known as the Chandler loop technique. It is not ideal as a large air/blood interface can cause protein aggregation and denaturation, creating a significant departure from the physiological situation. Furthermore, this method does not allow for arterial flow profiles to be obtained.
Another method for the investigation of flowing blood was the development of parallel-plate flow chambers. This apparatus is particularly useful in studying cellular interactions with a surface as the chambers are microscopically viewed in real time. However this is not helpful when studying actual coronary prosthetic configurations as the chambers and flow rates are not arterial in nature.
When studying coronary prosthesis, and in particular stents, one prior art method includes the use of a roller or peristaltic pump to drive flow through a length of tubing. The described setup utilizes a 3 mm ID, 82 cm long peristaltic tubing (PVC or silicon) filled with 6 ml of platelet rich plasma. A 3-way valve is used for the placement of fluid. The stent is expanded in a discontinuous connecting 4 mm ID segment. This methodology has recently been used to show variations in platelet activation, via Flow cytometry methodology and the clotting times for stents of different lengths and with heparin coatings, though it could not distinguish between tantilum and stainless steel stents. However, there are several factors that reduce the potential of this system to study stent thrombosis. One is the level of background noise that is created with the large surface area of peristaltic tubing and the roller pump's action. In order to keep the pump's background effects to a minimum, a low 8 ml/min flow rate was used, while actual mean flow rates of 50 ml/min are achieved in the coronary arteries with peak values normally reaching 100 ml/min. Furthermore, placing the stent in a discontinuous 4 mm region not only increases system background noise, but substantially perturbs the flow over the stent. Both the flow rate and stent placement create a dramatic misrepresentation of the dynamics of flow dependent thrombosis.
Another method that has recently been described as an in vitro evaluation of stent thrombosis includes a simple setup wherein blood is drained directly from a volunteer into a funnel connected to a length of tubing into which the stent is placed. The blood is directly collected into a tube and then analyzed for variations in platelet activation. This system reduces the background noise by using a shorter tubing length and no peristaltic pump. On the other hand, the signal is also reduced due to the one pass methodology rather than recirculation. Although some differences could be noted with certain stents, others were not significantly different than control runs, thus indicating the lack of sensitivity and that the flow rate was not controlled. Additionally, bleeding a volunteer requires a substantially greater amount of blood than recirculant setups.
Some animal in-vivo and ex-vivo models have been used. Although these have the ability to create physiological flows, they have a drawback in that there is a limit on the amount of control that is attainable in the system as parameter variation must be within life-sustaining margins. Therefore, studying the coupled nature of thrombus formation is difficult because the components cannot be varied to the extent that they may in an in-vitro setup. Many extraneous variables exist in in-vivo systems that could complicate the process being observed rendering unanalyzable results. Also interspecimen variation can create noise, which if large enough, could obscure potential findings. Another concern is that although observations may be made in one species, they may not be robust enough to occur in humans due to relative functional component differences. Practically, there are other issues, from the expense to the ethics, that must also be taken into account when using such systems. Though these issues limit what can be gained from in-vivo models, some studies have nonetheless been performed which are of relevance. For instance, Makkar et al., 1998, “Effects of lopidogrel, asprin, and combined therapy in a porcine ex-vivo model of high-shear induced stent thrombosis,” European Heart Journal. 19(10), 1538-1546 show in an ex-vivo pig model that polishing or polyethylene oxide modified nitinol surfaces were less thrombogenic than nitinol surfaces.
Other types of studies have included clinical trials. These carry with them many of the same problems as the animal studies. Additionally, there is even less controllability as the welfare of the patient is the primary concern, with many observations being taken retrospectively. Although in the end, these trials must be performed to validate findings from other models, the preliminary use of models can be used to investigate processes in a more scientifically rigorous fashion, while decreasing patient risk in clinical trials. Therefore, it is desired to develop a more suitable in-vitro model of the coronary situation to aid in the study of vascular phenomenon such as thrombosis.